In these days, general gas customer houses are attached with membrane gas meters, each of which measures a gas flow value based on the number of times the gas flows through a measuring chamber. Since the membrane gas meter requires a space used to provide therein the measuring chamber of a relatively large volume, in a measurement principle, it is difficult to further reduce the size of the membrane gas meter.
Under the circumstances, in recent years, to realize a small-sized gas meter, an ultrasonic gas meter has been developed. The ultrasonic gas meter includes ultrasonic sensors (transduces or transmitters/receivers) which are attached to the upstream and downstream sides of a fluid passage through which the gas flows, and is configured to measure the flow velocity of the gas flowing through the fluid passage based on the arrival time of an ultrasonic wave, and calculate the volumetric flow value of the gas based on the flow velocity of the gas, thereby deriving the usage amount of the gas. As should be understood, the ultrasonic gas meter is capable of measuring the usage amount of the gas so long as there is provided a fluid passage for measuring the flow value. Therefore, it is easy to reduce the size of the ultrasonic gas meter.
Also, there is a need for development of a gas meter which is capable of measuring the usage amount of the gas for facility such as a factory which consumes a gas of a large amount, instead of the general gas customer houses. As an example of such a gas meter which is capable of measuring a large gas usage amount, there is proposed a fluid meter device configured in such a manner that plural measurement fluid passages (measurement passages) connected to each other in parallel are provided, a pair of transducers (flow value measuring units) are provided in each of the measurement fluid passages, the flow values in separate measurement fluid passages are measured, and a total flow value of these flow values is decided as the flow value of the whole fluid meter device (see Patent Literature 1).